Precast concrete swimming pool



Sept. 1, 1959 E. L. CULVER 2,902,157

PRECAST CONCRETE SWIMMING POOL Filed Feb. 13, 1956 "r Sheets-Sheet 1 mmqllnmmwly l; 11ml INVENTOR EZ/"al. Uzilver WWW ATTORNEYJ P 1, 1959 E. L.CULVER 7 2,902,157

PRECAST CONCRETE SWIMMING POOL Filed Feb. 13, 1956 7 Sheets-Sheet 2INVENTOR Zizra LCzblver BY ZMg/W ATTORNEYJ Sept. 1, 1959 E; cu v2,902,157

v PRECAST CONCRETE SWIMMING POOL Filed Feb. 15, 1956 '7 Sheets-Sheet sINVENTOR Ezra LCuZver Sept. 1, 1959 E. L. CULVER PRECAST CONCRETESWIMMING POOL 7 Sheets-Sheet 4 Filed Feb. 13, 1956 lllllllllllllllllll-W llll m INVENTOR Ezra Z. Uzz/Zver Sept. 1, 1959 EL. CULVER PRECASTCONCRETE SWIMMING POOL Filed Feb. 15, 1956 7 Sheets-Sheet 5 INVENTOREzra Z. Culver A ORNEYS Sept. 1, 1959 E. L. CULVER PRECAST CONCRETESWIMMING POOL '7 Sheets-Sheet 6 Filed Feb. 13, 1956 z; LL16: QME

ATTORNEY? Sept. 1, 1959 E. L. CULVER 2,902,157

PRECAST CONCRETE SWIMMING POOL Filed Feb. 13, 1956 7 Sheets-Sheet 7Unite States Fate t PRECAST CONCRETE SWIMMING POOL Ezra L. Culver,Florence, Ala.

Application February 13, 1956, Serial No. 565,245

Claims. (Cl. 210-169) This invention relates to swimming pools, and moreparticularly to comparatively small swimming pools for private homes. Itis directed to the problem of installing a swimming pool in an averageresidential yard at reasonable cost.

Private swimming pools of this type, although smaller ice Fig. 11 is afragmentary section taken on line 11-11 of Fig. 10;

Fig. 12. is a partial side view and longitudinal section taken on line12-12 of Fig. 13, showing the pressure filter and associated equipment;

Fig. 13 is a plan view of the filter cover and associated equipment; and

Fig. 14 is a partial side view and transverse section taken on line14-14 of Fig. 13;

, Fig. 15 is a plan view of another precast swimming pool shell made inaccordance with my invention, made in two sections disposed on oppositesides of the longitudin'al axis, the filters, filter covers andassociated parts, which are disposed in and above filter cavities 206 inthe complete swimming pool, being omitted;

Fig. 16 is a side view of one section of the same, taken A on line 16-16of Fig. 1;

than public pools, are expensive to install, particularly if afiltration system is included. A reinforced concrete pool of family sizerequires considerable time and labor to install, and is a custom jobinvolving, besides excavation, expensive concrete and plumbing work. Theoverall costs of such pools are from $3,000 to $10,000.

My invention provides a comparatively light-weight concrete pool ofnovel construction which can be precast at the factory, equipped with anintegral filtration plant complete with all plumbing, transported overthe regular highway system to a selected location, and there installedin a suitable excavation, without the need for any extra plumbing. Mynovel structure preferably includes a pressure filter chamber castintegrally with the shell of the pool, and novel reinforcing means.which makes it possible to use comparatively thin, light concreteshells. This provides a life-time pool at substantially less total costthan conventional swimming pools.

A primary object of my invention is, therefore, to provide acomparatively inexpensive lightweight precast concrete swimming poolhaving an integral pressure filter chamber, and provided with a sand andgravel pressure filtration plant complete with plumbing. Another objectis to provide novel reinforcing means for concrete swimming pool-s.Another object is to provide a prestressed concrete swimming pool, i.e.,one which is stressed after it has been cast, and before it is subjectedto loads, such as those resulting from filling it, or stress loadscaused by transporting it. Another object is to provide novel means forprestressing a concrete basin or tank. Other objects will becomeapparent from the following description and from the drawings, in which:

Fig. 1 is a plan view of a precast swimming pool shell made inaccordance with my invention, the filter, filter cover and associatedparts (which are shown in Figs. 12-14) being omitted;

Fig. 2 is a vertical section taken on line 2-2 of Fig. 1;

Fig. 3 is a side view of the same;

Fig. 4 is a section taken on line 4-4 of Fig. 1;

Fig. 5 is an end view of the same, taken at the deep end;

Fig. 6 is a section taken on line 66 of Fig. 1;

Fig. 7 is an enlarged fragmentary view taken on line 77 of Fig. 1;

Fig. 8 is an enlarged fragmentary view taken on line 99 of Fig. 1;

Fig. 9 is an enlarged fragmentary section taken on line 9-9 of Fig. 1;

Fig. 10 is an enlarged fragmentary portion of Fig. 1 showing in placeportions of tension means which surrounds the filter cavity;

Fig. 17 is a section taken on line 1717 of Fig. 15;

Fig. 18 is an end view of the pool of Fig. 15, showing the deep end;

Fig. 19 is a section taken on line 1919 of Fig. 15

Fig. 20 is an enlarged fragmentary section taken on line 2020 of Fig.15;

Fig. 21 is an enlarged fragmentary section taken line 21--21 of Fig. 15;

Fig. 22 is an enlarged fragmentary section taken line 2222 of Fig. 15;

Fig. 23 is an enlarged fragmentary section taken line 2323 of Fig. 15

Fig. 24 is an enlarged fragmentary section taken line 24-24 of Fig. 15;

Fig. 25 is an enlarged fragmentary section taken line 25-25 of Fig. 15;and

Fig. 26 is a schematic drawing of the piping connections and equipmentof the pressure filter system of the embodiment of Fig. 15.

Referring now to the drawings, wherein like characters designate thesame or similar parts, there is shown in Figures 1 to 14 one embodimentof my invention comprising a precast, prestressed concrete swimming poolor tank generally designated 100, comprising a basin or concave shell101. Shell 101 is surrounded by a rim portion 102 at the top, from whichdepend side walls 103 on each side and end walls 104a and 10 1b at thedeep and shallow ends respectively. The bottom of the shell is closed bya sloping bottom 101a which slopes downwardly from shallow end wall10412 toward deep end wall 104a. The rim portion, side walls, end wallsand bottom wall are integral parts of the monolithic concrete shell wall101. A drain well or pool outlet 116 is provided at the deepest point.Deep end wall 104a has an enlarged wall portion 105 extending downwardlyfrom rim 102, and a deep filter cavity 106 extends downwardly in portion105. The side and bottom walls of filter cavity 106 are pressure tight.A filter drain line 109 extends from the bottom of cavity 106 throughthe concrete to a point within the basin or shell adjacent drain 116.This is normally closed by a plug 110. The filter cavity 106 has anenlarged portion 107 above it, an annular shelf or shoulder 10% beingprovided at the bottom of the upper cavity portion 107 surrounding thelower portion 106.

The Walls of cavity 106 may be strengthened by a spiral reinforcing rod106a embedded in the concrete. Filter cavity 106 contains a filter bed111 of sand and gravel, sand being in the upper portion and layers ofincreasingly coarse gravel being disposed below that to fill the majorportion of cavity 106, but leaving a small space at the top. A cover 112rests on annular shelf 108 and is held in sealing engagement therewithby a gasket 113 and suitable anchor bolts 114 and nuts, therebyproviding a pressure-tight cover for cavity 106. A supply pipelllaextends through the cover to the top portion of enclosed ..chamber 10.6,and a return line .1111: also .extends through the cover, from thebottom portion of the filter. Thus, closed cavity 106 constitutes anembedded or-integral .sand and graveLpressure'filter in. swimming pool100. Instead of using the cavity walls-themselves astiilter walls 1 mayembed .a filter tankdn: thescavity.

Mounted on topof cover 112 is a moton-diiven pump 115. Main drain lineor ,ssvupply pi-pe 117 .connects'the drain well .116 with ;the,.sucti;onsideof pump 115 through a stop valve, and a strainer 118 for catchingthair, lint, and the like. dischargepipe 111 9 connects with amulti-port valve 125. When valve 120 ;-is iniitsnormal position itchannels the Water irompump 115 ,to the top OL filter llieqthrough line111a. Waterfrom the pool passes down through the filter, is filtered,and emerges at the bottom into vertical tube 1111: which leads'it .upthrough the top of the tank and-backto multi-port valve 120 which, inthe normal position, channels the fiowgto outlet 122. Copper tubingreturnlpipes 1'23 extend from outlet pipe 122 in both directions, areembedded in the Walls .of shell 101, and vextend the length thereof ,on.both sides to-outlets 124 into theshcll at'the-shallowcnd, for return offiltered water to the pool proper. Main .drain or :returnpipe line 117is covered by a concrete rib. 11 7a which is cast integrally with theshell 1101.

It will thus be seen that the piping for circulating the water from thepool to the filter and from the filter back to the pool is embedded inthe pool shell, and that pool 100 has, as an integral, part, a completesand; and gravel pressure filter system.

To provide means forcleaning thepool with a-vacuum attachment, a vacuumline 126, provided with ,a suitable stop valve, also leads to the vacuumside .of pump'115 through strainer 118.

For backwashing the filter, the multi-port valve .120 is turned tobackwash position, which channels water taken from the pool through line117 into filter 10,6 via line 111b to the bottom of the filter. Thebackwash water then rises through the filter bed to ,clean=it,--and thiswater leaves through the top of the tank via ;;pipe 111a back to valve120 which diverts it out line "-125 .to Waste.

4 tion 105, for example to an embedded stiff ring such as ring 132.

Surrounding tension means 130 to 134 inclusive is placed close to thetop of shell 101 in order to be close to the stress-applying points ofthe transverse and longitudinal tension means described below.

If desired, one or more additional encircling tension means such asmembers 130 to 134 inclusive, with embedded ring or anchoring means likering 132 may be provided below the illustrated means for applyingadditional contractive forces to the shell. These may also behorizontally disposed, or they may be inclined downwardly .toward .the-deep end. Means are also z-provided to prestress .the shell .101transversely by tension Shell .101 is comparatively thin throughoutthetmajor portions of the bottom, side, .and end walls .(except :for theenlarged portion 105), being'about .two inches'thick except for'thethickened rim portion and ribs .described below. High quality, dense.concreteis .used; .e.g. 5000 p.s.i. (pounds .per square inch) concreteas compared to the 3000 psi. quality normally used for swimmingpoolspoured in situ. Means are provided for prestressing this comparativelythin dense shell before any loads are applied to it, so as to reinforceit and prevent it firom cracking, and these means will now be.described.

As will be noted in the vertical cross-sectional views of Figures 2, 4and 6, rim portion 102 is thickened fora short distance below the top ofthe pool. Within rim portion 102 is aconduitlSfi which extends cleararound the pool, its .ends terminating in upper cavity portion 107. Anopening 102a is .also provided'at .one point in rim 192 to accommodate aturnbuckle 13.4. .A stiff steel ring 132' is provided in cavity portion107. Leading outwardly fromring 132 in both direction and extendingthrough conduits 130 on both sides are two sections of a wireor rod 131.The ends of these sections .of rod 131 terminate in turnbuckle 134.Turnbuckle134,as well as nuts 133 adjacent ring 132, provide meanswhereby wire or rod 131 may be tensioned to apply an encirclingcompressive .or contractive force to rim portion 102.

It will be noted that ring 132 is mounted so as to be free of theconcrete of enlarged portion 105. Although this is preferred for theparticular shape of pool illustrated in'this embodiment, the ends ofwireor'rod 131 adjacent the filter may be anchored tosuitable'reinforcing means embedded in the walls of enlarged pormeanswhich extend along the shell Wall from side to side, either outside theshell, or within it. For this purpose in the illustrated embodimentthere are provided a plurality of transverse tension wires or rodshaving means for bearing downwardly against :the shell 101 at rimportion 1025011 each side. Tension wires '135 are spaced alongthe'longitudinal axistof the pool in vertical transverse planes.'Innthepreferred-form illustrated,. each wire or rod 135 is freelyslidable in a conduit 113. 6 embedded in a -rib 137 which is castintegrally with shell 10 1. Preferably, conduit 13 6 is not bonded totheconcrete'but iscoated with a substance such as asphalt,-to preventbonding. In addition, the upper .ends .of conduit 136 on each sidedo-notextend quite to the .top of=rib 137, so that a nut and washer 138, whenset up tight to tension rod 135, will not bear against conduit 136. Thepurpose of this is .to avoid resistance .to the prestressing orcompressingotf the shell when rods 135 are ,tensioned by setting .up onnuts 138.

Additional means are provided for applying compression to theshell:longitudinal'lyfrom the rim at .one .end to the rim atthe otherend. This rneansmay be disposed within the shell or outside it. Thepreferred arrangement is similar to that forlthetransverse tensionmeans, and comprises tension wires or rods 140 freely siidable in tubesor conduits 14 1 which are embedded vin but not bonded to the concreteof covering ribs 142, said ribs being ,cast integrally withthe shell101. Tension means such as a nut and washer 143 is provided at eachend-of each rod ll ldto apply downward or compressive force to the shellat rim-portion m2 at each end of the pool. A plurality of theselongitudinal tension means are provided --in vertical longitudinalplanes which are spaced transversely of the pool.

After the pool has been cast and has hardened the various tension wiresor rods are threaded'through the respective conduits and the nuts,Washers'and turnbolts placed in position. Tension is then graduallyapplied to each tension member so as touniformly and-gradually compressthe shell. The amount of tension applied depends upon the particularinstallation and is a matter determined by experience, but a forceofabout 45 pounds applied to the nuts and turnbuckle at the end of a71-foot lover (or torque of :90 pounds-feet) is satisfactory-in mostcases. When theshel l has been properly tensionedthe opening 102a in therimis filled witheoncrete, embedding turnbuckle 134.

Thus, it will be seen that the several tension means form a reinforcingsystem of prestressing members disposed along the contour of the shellwall for applying compressive forces to the shell in directions alongthe shell con-tour.

Enlarged wall portion with .its enclosed filtration plant may bediiferently shaped and disposed, if desired. For example, referring .totheplan view of the-pool, i-n stead of arranging the vertical axis ofthe filter inthe center of deep end wall 104a, with one side of thefilter wall extending into the pool proper and .the other side extendingoutwardly, enlarged portion 105 may be arranged inwardly or outwardly ofthe illustrated location with thefilter side Wall-tangentto orcoextensive with a flat end wall 104a. Such an arrangement avoids thenecessity of using a ring such as 132, or anchoring means in enlargedportion 105 for tension wire or rod 131, which then has a straight leadin wall 104a, so that wire 131 may be continuous all around the pool,from one end of the turnbuckle 134 to the other.

Although I prefer to use wires or rods slidable in tubes for theprestressing members, other means may be used. For example, theprestressing members may be coated with asphalt to prevent bonding toconcrete, and directly embedded in the walls during precasting. v

Another embodiment of my invention is shown in Figures 15 to 26. Thispool or tank is made of two precas't sections 200a and 2001) of 5000p.s.i. concrete which are assembled at the installation point. Asassembled, this pool, generally designated 200, comprises a concreteshell or basin having side walls 203, a deep end wall 204a, and ashallow end wall 20%. Bottom wall 201a and b slopes downwardly from theshallow end wall 204b toward the deep end wall 204a and has a drain orpool outlet 216 in the deepest portion. The side walls and part of thebottom walls are two inches thick, the rim portion and the bottomadjacent the abutting edges being thickened, and thickened rib portionsare also provided over the longitudinal and transverse prestressingmembers. Deep end wall 204a has an enlarged wall portion 205 in eachsection, and each enlarged wall portion has two cavities 206 extendingdownwardly therein, making a total of four such cavities 206, each ofwhich holds a filter bed similar to that illustrated and described forthe embodiment of Figures 1 to 14. Each of these filter cavities may besomewhat smaller than the single cavity of the first embodiment. Eachfilter cavity 206 has its own filter drain line 209 into the pool, whichis normally closed by a drain plug 210. Each cavity 206 has a cover 212,and each filter bed is provided with supply and discharge lines similarto that shown for the embodiment in Figures 12 to 14 at 111a and 1111).Each filter is also provided with its own multi-port valve 220, similarto valve 120 of the first embodiment. However, only one motor-drivenpump 215 is provided. Figure 26 shows a schematic of the pipingarrangements which draw pool water from pool outlet or drain 216,through supply pipe 217, through suction strainer 218 to pump 215 andthence to the multiport valves 220 through lines 219. In the normal orfilter position of valves 220, pool water enters the filters at the top,goes down through the filter bed and leaves through a tube leadingupwardly from the bottom, back through multi-port valves 220 and thencethrough connection 222 to the return tubing 223. Return tubing 223 leadsto four discharge inlets 224 into the pool, two at the shallow end, andtwo at the deep end. This tubing 223 is embedded in the pool wall. Thesupply pipe 217 from the pool drain is covered by a rib 217a which iscast integrally with one section of the shell of the pool. Thus, theplumbing for circulating the water from the pool to the filter and backagain to the pool is embedded in the shell of the pool, and pool 200has, as an integral part, a complete sand and gravel pressure filtersystem.

A vacuum line 226 is also provided to the suction side of pump 215. Thisline is normally covered by a cap 227, but a vacuum device may beattached to it for pool-cleaning purposes.

For backwashing the filter, valves 220 are thrown to the backwashposition, which reverses the flow through the filter and discharges thewater through line 225 to waste.

A reinforcing system is provided, similar to that of the firstembodiment described above, for prestressing or compressing the shell.Horizontal or encircling means 230, 231, and 234a in rim portion 202correspond to means 130, 131, and 134, respectively, but no ring such as132 is required, the encircling member being disposed in the wall 204abetween the filters and the pool proper.

A second horizontal encircling member is spaced beldw the upper means,and has its own turnbuckle 234b.

Transverse tension means 235, 236, and 238, disposed in ribs 237,correspond to means 135, 136, 138, and 137, respectively. Longitudinaltension means 240, 241, and 243 disposed in ribs 242, correspond tomeans 140, 141, 143, and 142, respectively.

:In pool 200, prestressing members 231 and 235 serve not only tocompress the shell walls, but to draw the two sections tightly together.The tubes or conduits 230 and 236 are, of course, each in two parts, oneembedded in one pool section 2000 and the other in pool section 20012.The two parts of each conduit are carefully located in their respectivepool sections so that, when the pool is assembled, their adjacent endswill be alined.

To provide efficient sealing, grooves 249 are provided in the abuttingedges of the two halves of the pool, and live rubber gasket 250extending throughout the lengths of these grooves insures propersealing. In addition, caulking 251 may be provided above the gasket.

To assemble pool 200, the two sections are placed closely adjacent eachother and the various prestressing members are threaded through theirrespective conduits and the turnbuckles and nuts and washers placed ontheir ends. The gasket 250 is placed in position. By getting up on theturnbuckles and nuts gradually to a torque of about 90 pounds-feet thetwo sections are drawn tightly together in correct minement, with gasket250 between them to seal the pool, and with the shell properlyprestressed. Access openings in the shell around turnbuckles 234a and23411 are then filled with concrete.

Although not essential, transverse rods 252 and 253 are provided on eachside of the bank of filters, in order to compress and give additionalrigidity and support to this portion of the structure.

In both the illustrated embodiments, the ribs on the outer sides of theshell (137 and 142, or 237 and 242) provide reinforcement to the shelland make possible the use of thinner shells which weigh less, inaddition to providing efficient means for mounting the tension means forcompressing the shell. However, the ribs are not essential, and thetensioning means may be stretched along the outer surface of the shellor be enclosed within the walls of the shell, the ends of the transverseand longitudinal tension members being provided with suitable means totension the same and pull down on the rim of the shell.

The longitudinal prestressing means passing under the pool from the pooledge at one end to the pool edge at theother end may be dispensed within some pool configurations, especially if the pool is comparativelylong and narrow and of more uniform depth than those illustrated. Inthis case, more encircling tension wires or rods are used, the lowerones being inclined downwardly toward the deep end so to give betterdistribution of the encircling reinforcement around the deep end.

Although the preferred method of reinforcing precast pools made inaccordance with my invention is to prestress or compress the shell asdescribed above for the two illustrated embodiments, I may also make aprecast swimming pool with integrally cast filter chamber, and withinstalled sand and gravel pressure filter system complete with plumbing,using embedded, unstressed reinforcing. In this form, the walls arereinforced by steel wire mesh, preferably 4" x 4" No. 6 electric weldedsteel wire mesh, and the ribs on the outer side of the pool (which coverthe prestressing means in the two illustrated embodiments) are omitted.

I may use my preferred reinforcing system of prestressing members forreinforcing the concrete basins or shells of swimming pools or liketanks which do not have integral filters.

For comparatively small swimming pools, ranging up to about fifteen bythirty feet, I prefer to use the first embodiment, designated 100. Forlarger sizes I prefer ,the second embodiment 20.0 .ora pool madeof morethan twodarge sections.

"The method ,of assembly of both embodiments has been described above.To prevent cracking during transportation, pools 109 and 2% arethoroughly prestressed beforelbeing moved. In the case of pool 20% thetwo sections are, of course, separately prestressed, utilizing theembedded conduits in each section for running special stressed rods orwires. In installing either embodiment, a suitable excavation isprepared, slightly wider than the pool. A sand bed is laid on the bottomof the excavation and the pool is moved into place from the shallow endwith suitable handling equipment. The pool is trued and -levelled on thesand bed. Earth is then back-filled all around the pool slowly, withtamping, while the pool is filled a little at a time, thewater levelbeing kept at the height of the fill, or slightly above. No undergroundpiping to or from the pool is necessary, hoses being used to fill thetank directly, or to drain it from the back-wash line.

For the tubes or conduits through which the prestressing members extendI prefer to use flexible conduit, :such as standard one-inch BX electricconduit. For the prestressing members I prefer to use one-half inch highcarbon steel rod. In the appended claims the term wire is to be broadlyinterpreted and includes rod as well as wire.

The advantages of my precast concrete swimming pools will be apparent tothose skilled in the art. Fittings, tubes, and piping are cast in placein the concrete shell, and a complete unit may be assembled, ready forinstallation, thereby saving the usual costs of custom installation. Thecost of a pool of my improved type, installed, is less than that ofcomparable installations with which I am familiar, and my pools can bemass-produced.

Numerous changes within the scope of the appended claims will beapparent to those skilled in the art. For example, any prestressingmember may be anchored to the shell intermediate its ends, or may bemade in sections with the adjacent ends anchored to the shell, and bothportions or sections tensioned so as to provide a substantiallycontinuous prestressing member.

I claim:

1. A transportable swimming pool comprising a precast concrete shell,said shell being concave with a rim portion around the top and having apair of opposite end walls depending from said rim portion disposedtransversely of the longitudinal axis of said pool and a pair ofopposite side walls depending from said rim portion, one on each side ofsaid axis, one of said walls having an enlarged wall portion and acavity extending downwardly therein, a pressure-tight filter tank insaid cavity, said filter tank comprising a pressure-tight cover spacedabove the bottom of said cavity, said cover enclosing the portion ofsaid cavity below said cover and forming the top of said pressure-tightfilter tank, a filter supply pipe embedded substantially throughout itslength in said shell and extending therealong from an outlet from saidshell to a point adjacent said tank, a filtered-water return pipeembedded substantially throughout its length in said shell and extendingfrom a'point adjacent said tank to an inlet into said shell, said tankhaving a filter therein, said pool having a circulating system includinga pump and said supply and return pipes for circulating water from'saidshell to said filter tank through said filter and back to said shelLashell reinforcing system of prestressing members disposed along thecontour of said shell and applying prestressing compressive forcesthereto in directions along said contour, said system comprising anencircling member around said pool applying a contractive force to saidrim portion, a plurality of longitudinal tension members disposedtransversely to said encircling member and extending along said shellfrom the rim portion at one of said'end walls to the rim portion at theopposite of said end walls, and a plurality of transverse tensionmembers disposed transversely to said encircling member and extendingalong said shell from the rim portion at one of said side walls to therim portion at the opposite of said side walls, a plurality of concreteribs cast integrally with said shell on the outer side thereof, eachsaid rib covering one of said tension members, said rib terminating inopposite end bearing surfaces, each of said bearing surfaces beingadjacent said rim portion on opposite sides respectively of said shell,said one of said tension members being slidably disposed in a channelextending along said shell, and means on said one of said tenion membersapplying a compressive force against said opposite end bearing surfacesof said rib to apply a prestressing force to said shell wall; wherebysaid swimming pool comprises a prestressed concrete swimming poolcomplete with filtration plant which can be precast, assembled, andprestressed in one location, transported to another location, and thereinstalled in a suitable excavation.

2. A transportable swimming pool comprising a precastreinforced-concrete shell, said shell comprising a concave shell wallwith a rim portion around the top, a system of prestressing membersdisposed along the contour of said shell wall and applying compressiveforces thereto in directions along said contour, said system comprisingan encircling member around said pool applying a prestressingcontractive force to said rim portion, and a plurality of transversetension members disposed transversely .to said encircling memberextending along said shell wall from the rim portion on one side of saidshell to the rim portion on the opposite side thereof, said transversetension members applying prestressing compressive forces to said shell,at least one of said transverse tension members being covered by aconcrete rib cast integrally with said shell wall on the outer sidethereof, said rib terminating in opposite end bearing surfaces, each ofsaid bearing surfaces being adjacent said rim portion on opposite sidesrespectively of said shell, said one of said tension members beingslidably disposed in a channel extending around said shell and havingmeans applying a compressive force against said opposite end bearingsurfaces of said rib to prestress said shell wall, said shell wallhaving an enlarged wall portion and a cavity extending downwardlytherein, a pressure-tight filter tank in said cavity, said filter tankcomprising a pressure-tight cover spaced above the bottom of saidcavity, said cover enclosing the portion of said cavity below said coverand forming the top of said pressure-tightfilter tank, said pool havinga circulating system for circulating water from said shell'through saidfilter tank and back to said shell; whereby said swimming pool may beprecast and completely assembled in one location, transported to anotherlocation, and there installed in a suitable excavation.

3. A transportable swimming pool comprising a precast concavereinforced-concrete shell, a system of prestressing members disposedalong the contour of said shell and applying compressive forces theretoin directions along said contour, said system comprising an encirclingmember around said pool applying a prestressing contractive force tosaid rim portion, and a plurality of transverse tension members disposedtransversely to said encircling member extending along said shell fromthe rim portion on one side ofsaid shell to the rim portion on theopposite side thereof, said transverse tension members applyingprestressing compressive forces to said shell, each said transversetension member being covered by a concrete rib cast integrally with saidshell on the outer side thereof, said rib terminating in opposite endbearing surfaces, each of said bearing surfaces being adjacent said rimportion on opposite sides respectively of said shell, said tensionmembers being slidably disposed in a channel extending around said shelland having means applying a compressive force against said opposite endbearing surfaces of said rib to prestress said shell, said shell havingan enlarged wall portion and a cavity extending down- Wardly therein, apressure-tight filter tank in said cavity, said filter tank comprising apressure-tight cover spaced above the bottom of said cavity, said coverenclosing the portion of said cavity below said cover and forming thetop of said pressure-tight tank, a filter supply pipe embeddedsubstantially throughout its length in said shell and extendingtherealong from an outlet from said shell to a point adjacent said tank,and a filtered water return pipe embedded substantially throughout itslength in said shell and extending from a point adjacent said tank to aninlet into said shell, said tank having a filter therein, said poolhaving a circulating system including a pump and said supply and returnpipes for circulating water from said shell to said filter tank throughsaid filter and back to said shell; whereby said swimming pool may beprecast and completely assembled in one location, transported to anotherlocation, and there installed in a suitable excavation.

4. A prestressed concrete tank comprising a concave shell wall having arim portion around the top, an encircling tension member disposed alongsaid rirn portion and applying a prestessing contractive force thereto,and a plurality of transverse tension members disposed transversely tosaid encircling tension member along said shell wall substantially fromthe rim portion on one side of said shell to the rim portion on theopposite side thereof, said transverse tension members applyingprestressing compressive forces to said shell transversely to said rimportion, whereby said encircling and transverse tension members coact toapply prestressing compressive forces to said shell substantially allalong the contour thereof, at least one of said transverse tensionmembers being covered by a concrete rib cast integrally with said shellWall on the outer side thereof, said rib terminating in opposite endbearing surfaces, each of said bearing surfaces being adjacent said rimportion on opposite sides respectively of said shell, said one of saidtension members being slidably disposed in a channel extending aroundsaid shell and having means applying a compressive force against saidopposite end bearing surfaces of said rib to prestress said shell wall.

5. A prestressed concrete tank comprising a concave shell wall having arim portion around the top, said shell wall having a pair of oppositeend wall portions depending from said rim portion disposed transverselyof the longitudinal axis of said tank and a pair of opposite side wallportions depending from said rim portion, one on each side of said axis,an encircling member around said tank applying a prestressingcontractive force to said rim portion, a plurality of longitudinaltension members disposed transversely to said encircling memberextending along said shell wall substantially from the rim portion atone of said end wall portions to the rim portion at the opposite of saidend wall portions, and a plurality of transverse tension membersdisposed transversely to said encircling member extending along saidshell wall substantially from the rim portion at one of said side wallportions to the rim portion at the opposite of said side wall portions,said longitudinal and transverse tension members applying prestressingcompressive forces to said shell transversely to said rim portion,whereby said encircling and transverse tension members coact to applyprestressing compressive forces to said shell substantially all alongthe contour thereof, each said longitudinal and transverse tensionmember being covered by a concrete rib cast integrally With said shellWall on the outer side thereof, said rib terminating in opposite endbearing surfaces, each of said bearing surfaces being adjacent said rim.portion on opposite sides respectively of said shell, said tensionmember being slidably disposed in a channel extending around said shelland having means applying a compressive force against said opposite endbearing surfaces of said rib to prestress said shell Wall.

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